1. Technical Field
The present invention generally relates to a jet fuel having improved thermal-oxidation stability, as determined by ASTM D3241, and methods for preparing same.
2. Description of the Related Art
Distillate fuels which are intended for use in jet turbines must meet certain minimum standards in order to be suitable for use. Jet fuel must have good oxidation stability in order to prevent the formation of unacceptable amounts of deposits which are harmful to the turbine engines in which they are intended to be used. Jet fuel is also used as a heat sink in turbine engines. These deposits will create maintenance problems in the turbine engines.
Currently, fuel thermal stability is recognized as one of the most important properties of jet fuels. ASTM D3241 is the standard analytical procedure for rating fuel thermal stability and a fuel will either pass or fail at a given temperature. Generally, fuels for use in jet turbines will usually have a passing jet fuel thermal-oxidation tester (JFTOT) rating as determined by ASTM D3241 at 260° C.
The thermal stability of jet fuel has been recognized as a problem for many years. High speed flight necessitates that the heat generated be dissipated through the fuel, i.e., the fuel is purposely preheated prior to combustion. As aircraft have become more sophisticated with more electronic components, the heat load has increased and the fuel must be preheated to a higher temperature to absorb the energy. This makes the thermal stability of the fuel even more critical. The chemistry leading to particulate and deposit formation is extremely complex and very difficult to provide thermal stability of jet fuels.
The impact of lowering the aromatic content of distillate fuels used as diesel fuel or jet fuel on seal swell in diesel and jet engines is known. Since the transition from conventional distillate fuels to low aromatic fuels created problems with seal swell, greater seal swell problems associated with the transition to a highly paraffinic distillate fuel component made from, for example, a Fischer Tropsch process, is expected.
Distillates having very high levels of saturates, such as distillates recovered from the Fischer Tropsch process, have been shown to have excellent smoke points, usually in excess of 40 mm, and low sulfur contents. As such, highly paraffinic distillates appear to be useful for blending with lower quality distillates in order to obtain a distillate blend meeting the requirements for jet fuel. What has not been recognized is that some highly paraffinic distillate components, especially those characterized by low to moderate branching of the molecule, such as those products produced by the low temperature Fischer Tropsch process, when blended with conventional distillate components can show poor thermal stability leading to the formation of unacceptable amounts of deposits.
U.S. Pat. No. 4,162,961 discloses a cycle oil that is hydrogenated under conditions such that the product of the hydrogenation process can be fractionated.
U.S. Pat. No. 4,968,402 discloses a one stage process for producing high octane gasoline from a highly aromatic hydrocarbon feedstock.
U.S. Pat. No. 5,219,814 discloses a moderate pressure hydrocracking process in which highly aromatic, substantially dealkylated feedstock is processed to high octane gasoline and low sulfur distillate by hydrocracking over a catalyst, preferably comprising ultrastable Y and Group VIII metal and a Group VI metal, in which the amount of the Group VIII metal content is incorporated at specified proportion into the framework aluminum content of the ultrastable Y component.
U.S. Pat. No. 5,468,262 discloses the use of phenol-aldehyde-polyamine Mannich condensate with a succinic acid anhydride bearing a polyolefin to improve the thermal stability of jet fuel at 260° C.
U.S. Pat. No. 5,868,921 discloses a hydrocarbon distillate fraction that is hydrotreated in a single stage by passing the distillate fraction downwardly over a stacked bed of two hydrotreating catalysts.
U.S. Pat. No. 6,893,475 disclose a distillate fuel having a sulfur level of less than about 100 wppm, a total aromatics content of about 15 to 35 wt. %, a polynuclear aromatics content of less than about 3 wt. %, wherein the ratio of total aromatics to polynuclear aromatics is greater than about 11.
Accordingly, it would be desirable to provide a jet fuel composition having improved thermal-oxidation stability, as determined by ASTM D3241, as well as acceptable seal swell properties.